More testing/functionality for abinit headers

[brainandforce]

Although Electrum.read_abinit_header() is not exported, it is now easier to use, as it accepts a generic file name/path argument as well as an I/O handle. I've also defined equality for Electrum.ABINITHeader objects, and fixed setindex!() for them.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.63 :tada:

Comparison is base (dcaab94) 49.13% compared to head (3e6c64b) 49.77%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #164 +/- ## ========================================== + Coverage 49.13% 49.77% +0.63% ========================================== Files 17 17 Lines 1337 1328 -9 ========================================== + Hits 657 661 +4 + Misses 680 667 -13 ``` | [Impacted Files](https://app.codecov.io/gh/brainandforce/Electrum.jl/pull/164?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brandon+Flores) | Coverage Δ | | |---|---|---| | [src/software/abinit.jl](https://app.codecov.io/gh/brainandforce/Electrum.jl/pull/164?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brandon+Flores#diff-c3JjL3NvZnR3YXJlL2FiaW5pdC5qbA==) | `46.39% <100.00%> (+2.49%)` | :arrow_up: |

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